This invention relates to adaptive braking systems for wheeled vehicles, such as automobiles and the like, and more particularly to improvements in their logic systems which will eliminate or alleviate the condition known as wheel (or axle) wrap-up.
In adaptive braking systems using logic circuits to control vehicle braking during incipient wheel skid conditions, at least one sensor means is ganged to a portion of the vehicle power train such as a wheel, drive, shaft, speedometer cable, etc. and used to generate a signal proportional to rotational dynamic performance of the portion such as speed or acceleration.
In adaptive braking systems using speed logic, the dynamic performance sensed is speed and the sensor means generates a speed signal which is related to the instantaneous rotational speed of the rotating structure. Means are provided in the speed logic for memorizing a reference signal also related to speed but generally offset somewhat from the instantaneous speed represented by the speed signal. The logic is arranged so that the reference speed signal will track the speed signal, except that the rate of decay of the reference speed signal is limited. Thus, if after the vehicle brakes are applied, the speed signal should decay faster than the reference speed signal can follow, the speed represented by the speed signal will become equal to the speed represented by the reference speed signal. At that time or at a time related thereto the adaptive braking system will automatically operate to release braking force. However, due to inherent system delays, the speed and hence the speed signal will continue to decay so that it will represent a speed less than the reference speed. Eventually, in response to reduced braking pressure, the speed and hence the speed signal will begin to increase until the speed signal will again become equal to the reference speed signal. At that time or at a time related thereto the adaptive braking system operates to reapply braking force. Again, due to inherent delays in the system, speed of the sensed structure will not immediately begin to decrease in response to reapplied braking force but rather will continue to increase for some time. The speed signal, of course, follows the increasing speed and the reference speed signal also increases in response thereto. Subsequently, the inherent system delays are overcome and the restored braking force becomes effective to cause the sensed structure to again begin to slow down. At this time the phenomenon which has come to be known as wheel wrap-up occurs and which is characterized by sudden and short bursts of rapid wheel deceleration and subsequent acceleration accompanied by rapid fluctuations of the speed signal. Since the reference speed signal tracks the speed signal, this rapid pulsating of the wheel speed can cause the adaptive braking system to operate to release and restore braking force alternately in short cycles. This pulsating of braking force is undesired and normally results in reduced braking efficiency. It is thus an object of this invention to provide means to prevent certain causes of unnecessary and undesirable braking force fluctuations in an adaptive braking system operating in accordance with speed logic.
In adaptive braking systems using acceleration logic, the dynamic performance sensed is acceleration. In certain systems speed may be actually sensed and acceleration derived therefrom; however, in any event, at least acceleration of the sensed structure is considered by the logic section of the adaptive braking system to effect automatic control of braking force. This is accomplished by providing a first reference signal corresponding to some value of negative acceleration (deceleration) an comparing the acceleration signal against the first reference signal. When these two signals attain some predetermined relationship with each other, usually equal to each other, the system operates to at least initiate attenuation of braking force. Thereafter and in response thereto, the acceleration of the vehicle wheels will change. When a predetermined subsequent change has occurred the system operates to restore braking force. As discussed with respect to speed logic adaptive braking systems, wheel wrap-up at this time causes sudden and short bursts of rapid wheel deceleration and subsequent accelerations. This rapid pulsation can force the adaptive braking system to release and restore braking force alternatively in short cycles causing reduced braking efficiency. It is an object of this invention to provide means to alleviate the effects of wheel wrap-up in acceleration controlled adaptive braking systems.